1. Field of the Invention
The present invention relates to a process for hydroxylating aromatic compounds and more particularly a process for hydroxylating phenols and phenol ethers by means of hydrogen peroxide.
2. Description of the Prior Art
Numerous processes for the oxidation of phenols and phenol ethers by means of hydrogen peroxide together with metal salts, or by means of organic per-acids (formed from hydrogen peroxide and a carboxylic acid) have been described. Depending on the particular case, these processes have made it possible to introduce a hydroxyl group into the nucleus of the aromatic compound or have resulted in a more or less extensive oxidation of this nucleus, ranging from the production of quinones to the opening of the benzene ring, with the formation of degradation products[cf. A. CHWALA et al. J. Prakt. chem. 152 46 (1939); G. G. HENDERSON et al., J. Chem. Soc. 91 1659-69 (1910); S. L. FRIESS et al., J. Am. Chem. Soc. 74 1305 (1952); H. FERNHOLZ, Chem. Ber. 87 578 (1954); H. DAVIDGE et al., J. Chem. Soc. 1958 4569; J. D. McCLURE et al., J. Org. Chem. 27 627-8 (1962)]. None of these processes is of industrial value, either because the yields of hydroxylation products are poor or because the reaction conditions are unsuitable for industrial exploitation or because the oxidizing agent is inconvenient or even dangerous to employ.
U.S. Pat. No. 3,514,490 has proposed carrying out the hydroxylation of phenol by means of per-acids (performic acid) in the presence of phosphoric acid. Although this process represents an advance relative to those described before, it remains necessary to use relatively large amounts of carboxylic acids, which play a part in requiring an increase in the volume of the apparatus; the same is true of all the processes which employ the carboxylic per-acids, such as those described in French Pat. No. 2,146,195 and in Belgian Pat. No. 809,204. For this reason, attempts have been made to dispense with the use of per-acids and to employ hydrogen peroxide directly as the hydroxylating agent. Thus, U.S. Pat. No. 3,407,237 has proposed hydroxylating aromatic compounds by means of hydrogen peroxide in the presence of hydrofluoric acid; in fact, the hydrofluoric acid acts as the reaction medium, so that this process cannot be put into operation industrially.
Ultimately, none of the above-mentioned processes has made it possible industrially to carry out the hydroxylation of aromatic compounds by means of hydrogen peroxide and especially the hydroxylation of phenol to give hydroquinone and pyrocatechol. U.S. Pat. No. 3,849,502 has proposed a process for hydroxylating aromatic compounds, and more particularly phenol and its ethers, by means of hydrogen peroxide, alone, the characteristic feature of which resides in the fact that the reaction is carried out in the presence of a strong acid used in catalytic amount and, if appropriate, in the presence of a complexing agent for transition metals, such as ortho-, pyro- or poly-phosphoric acids or their esters, the initial water content, by weight, of the reaction mixture being less than 20% of the mixture of aromatic compound, water and hydrogen peroxide. This process represents a solution of the problem of hydroxylation of aromatic compounds, especially of phenol, on an industrial scale by virtue of its simplicity (in practice it requires only the use of catalytic amounts of strong acids [for example, perchloric acid, sulphuric acid or sulphonic acid, and of metal complexing agents (pyrophosphoric acid)] and because of the excellent yields of hydroxylation products which it provides. However, the achievement of these yields also depends on the degree of conversion of the aromatic compounds, which must be less than 30%, the value being controlled by an appropriate choice of the molar ratio of hydrogen peroxide to phenol. Preferably, the degree of conversion is restricted to a value of at most 15% and in practice values of 4 to 10% are not exceeded. Under these conditions, a limitation in the productivity of the apparatus results, because the process requires that the conversion of the aromatic compound should only be very limited and, separating off the hydroxylation products, a large volume of starting reactants should be recycled. It is thus important to ensure that the reaction should take place at as high a rate as possible so as to limit, to the maximum degree, the residence time of the reactants in the apparatus.
For given conditions of temperature and for a given water content of the medium, the rate of reaction depends on the nature of the acid, and for a given acid, on the amount of acid present in the reaction mixture. Regardless of the acid used, it appears desirable to be able to increase the rate of reaction without increasing the amount of acid and, preferably, even with a decrease in this amount. In practice, the acid used as the catalyst is extracted from the reaction mixture by washing with water, the wash waters being discarded as effluents after treatment, so that the acid catalyst is not recovered. It is thus important, from an industrial point of view, to be able to increase the productivity of the apparatus by increasing the rate of the reaction without increasing the required amount of catalyst and even with a reduction in this amount.